1. Field of the Invention
The present invention relates to a sampling frequency converter for converting a digital signal of a first sampling frequency into a digital signal of a second sampling frequency and, more particularly, to a sampling frequency converter adapted for use in conversion of the sampling rate or the like of a digital color video signal.
2. Description of the Prior Art:
In digital color signal formats, there is a known 4:2:2 format, also termed D-1 format which employs a digital component signal such that a luminance signal Y has a sampling frequency f.sub.1 of 13.5 MHz, while each of color difference signals R-Y and B-Y has a sampling frequency of 6.75 MHz which is equal to one half of the frequency f.sub.1. In contrast therewith, a composite color signal in the NTSC format, which is directly digitized and has a sampling frequency f.sub.2, set to an integral multiple of a color subcarrier frequency f.sub.SC, e.g. 4f.sub.SC (.apprxeq.14.318 MHz). Therefore to execute signal conversion between these signals, it is first necessary to execute sampling frequency (sampling rate) conversion between the two frequencies f.sub.1 and f.sub.2.
The R-Y and B-Y signals of the 4:2:2 format are both obtained by sampling at the frequency f.sub.1 /2; whereas the composite signal of the NTSC format is obtained by superimposing on the Y (luminance) signal, the carrier color signal produced by a quadrature two-phase modulation of the color subcarrier with the I and Q signals. Therefore, when the signal produced by sampling such a composite signal at the frequency f.sub.2 (=4f.sub.SC) is color-decoded, the I and Q signal data are obtained alternately per period 1/f.sub.2 (per 90.degree. of color subcarrier) as will be described later. That is, the decoded digital I and Q signals are such that the sampling frequency thereof is f.sub.2 /2 (=2f.sub.SC) and an offset corresponding to 1/f.sub.2 =1/4 f.sub.SC (90.degree. phase of color subcarrier) is existent therebetween. This phase difference between the sampling points of the I and the Q signal data necessitates interpolation to obtain components coincident in timing with the I and Q signals before or after conversion of the sampling frequencies when calculating the R-Y and B-Y signals from the I and Q signals by matrix operations.
FIG. 1 shows an exemplary conversion apparatus designed for converting a digital composite signal in the NTSC format into digital component signals in the 4:2:2 format (D-1 format). The NTSC formatted digital composite signal (sampling frequency f.sub.2 =4f.sub.SC) is fed to an input terminal 101 in FIG. 1 and is separated into a Y.sub.NT signal and a C.sub.NT signal (where the subscript NT denotes the NTSC format) by a digital Y/C separator 102. As shown in FIG. 2, the Y.sub.NT signal is composed of a sample data row having the aforementioned frequency f.sub.2 (=4f.sub.SC) which corresponds to a 1/4f.sub.SC period. The Y.sub.NT signal is fed to a sampling frequency converter (sampling rate converter) 103 and is thereby converted into a luminance signal Y.sub.D1 with the aforementioned D-1 standard sampling frequency f.sub.1 (=13.5 MHz). Such luminance signal Y.sub.D1 is taken out from a Y output terminal 104. Meanwhile, the C.sub.NT signal obtained from the Y/C separator 102 is fed to a decoder 107, which outputs decoded digital I.sub.NT and Q.sub.NT signals. The I and Q signals are decoded in the following manner. The original analog carrier color signal C in the NTSC format is expressed as: EQU C=I cos (.OMEGA..sub.SC t +.PHI.)+Q sin (.OMEGA..sub.SC t+.PHI.)
where EQU .OMEGA..sub.SC =2 .pi. f.sub.SC
Suppose that the phase (or the position on the time base), in sampling at the frequency f.sub.2 (=4f.sub.SC), increases stepwise from 0 with a unitary angle of .pi./2 (=90.degree.) as 0, .pi./2, .pi.. . . and so forth. The digital carrier color signal C.sub.NT itself is then changed as I, Q, -I, -Q and so forth per 1/4f.sub.SC. Therefore, in the decoder 107, the data row of the input color signal C.sub.NT is separated into I and Q per 1/4f.sub.SC (with period 1/2f.sub.SC) while being sequentially multiplied by .+-.1, thereby producing signals I.sub.NT and Q.sub.NT as shown in FIG. 2. Both I.sub.NT and Q.sub.NT have the same sampling frequency 2f.sub.SC and the time difference (offset) between the data of these signals is equal to 1/4f.sub.SC. Since it is impossible to execute a matrix calculation for obtaining R-Y, B-Y and so forth by the use of such I and Q signal data with different sampling points, the data at the sampling points (denoted by x on the signals I.sub.NT and Q.sub.NT in FIG. 2) of the other signals are interpolated by means of interpolators 108 and 109. This produces signals I.sub.f2 and Q.sub.f2 (shown in FIG. 2) having the same frequency F.sub.2 (=4f.sub.SC) with mutually equal sampling points. A matrix calculation, using such signals If.sub.2 and Qf.sub.2, is executed in a matrix calculator 110 producing signals (R-Y)f.sub.2 and (B-Y)f.sub.2 having the same sampling frequency f.sub.2 (=4f.sub.SC). Such signals (R-Y)f.sub.2 are converted by sampling frequency converters 111 and 112, respectively, into signals (R-Y).sub.D1 and (B-Y).sub.D1 at a mutually equal sampling frequency, f.sub.1 /2, as shown in FIG. 2, and then are taken out from output terminals 113 and 114, respectively.
FIG. 3 shows an exemplary format conversion circuit for converting a signal in the D-1 format (4:2:2 format) into a digital composite signal in the NTSC format, which is an inverse operation, with respect to the above. A signal Y.sub.D1 fed to an input terminal 121 is supplied directly to a sampling frequency converter 122 where the frequency f.sub.1 is converted into a frequency f.sub.2, changing the signal Y.sub.D1 to a signal Y.sub.NT which is sent to an adder 123. The color difference signals (R-Y).sub.D1 and (B-Y).sub.D1 supplied to input terminals 131 and 132, respectively, are fed to sampling frequency converters 133 and 134, respectively where the frequency f.sub.1 /2 is converted into a frequency f.sub.2 /2 (=2f.sub.SC) and then fed to interpolators 135 and 136, respectively, to become signals (R-Y).sub.f2 and (B-Y).sub.f2 having a sampling frequency f.sub.2 (=4f.sub.SC). Said signals are fed into a matrix calculator 137 becoming signals I.sub.f2 and Q.sub.f2 with a sampling frequency f.sub.2 (=4f.sub.SC), which are fed into a modulator 138 where a signal with a frequency f.sub.SC is modulated producing a digital carrier color signal C.sub.NT. Said modulation is performed sequentially by repetitively substituting values (1, 0), (0, 1), (-1, 0) and (0, -1) per sampling period 1/4f.sub.SC for the cosine value and the sine value in the aforementioned analog carrier color signal expressed below: EQU C=I cos (.OMEGA..sub.SC t+.PHI.)+Q sin (.OMEGA..sub.SC t+.PHI.)
where EQU .OMEGA..sub.SC =2.pi.f.sub.SC.
Said modulation produces I and Q signal data which appear alternately per sampling period 1/4f.sub.SC. The obtained digital carrier color signal C.sub.NT, with sampling frequency 4f.sub.SC, is fed to adder 123 and is thereby superimposed on the digital luminance signal Y.sub.NT, producing an NTSC digital composite signal with sampling frequency 4f.sub.SC which is taken out from an output terminal 124.
Since the above-described format conversion apparatus requires both interpolators and sampling frequency converters this produces problems in that the characteristic signal becomes somewhat deteriorated during each signal processing stage and also adds to the complexity of the circuit configuration.
When a composite signal of the NTSC format is converted into component signals of the 4:2:2 format as shown in FIG. 1, a discrepancy between the group delays of the luminance signal Y.sub.NT and the chrominance signal C.sub.NT results. This is partly because the respective frequency characteristics of the circuits for the luminance signal Y.sub.NT and the chrominance signal C.sub.NT are different, hence the filter characteristics for the sampling frequency conversion are also mutually different. Furthermore, due to decoder 107 and interpolators 108 and 109, the processing time required for the chrominance signal is longer than the luminance signal line, inducing a further difference between the respective group delay characteristics. The maximum error caused in correcting the discrepancies between the group delays of the two signals, by use of an ordinary sample delay method of similar means, is .+-.T.sub.1 /2, approximately 37 ns when the sampling period on the output side of the 4:2:2 format is set to T1, which is approximately 74 ns since f.sub.1 =1/T.sub.1 =13.5 MHz. Accordingly, some harmful influence such as color deviation may occur in the reproduced image. Likewise, in converting 4:2:2 format into the NTSC format as shown in FIG. 3, a group delay problem may arise between the luminance signal and the color difference signals due to the influence of the video signal processing characteristics and the recording and reproducing characteristics. This group delay discrepancy may exert some harmful influence on the image should the correction be performed merely per sampling period since the error is not sufficiently diminished as to be permissible.